1. Field of the Invention
This invention relates to operational amplifier circuits, and to more complex circuits employing operational amplifiers.
2. Description of the Related Art
Operational amplifiers (op amps) have become a universal building block for analog systems. The principal reason for this is the ease with which their overall characteristics can be accurately defined using simple feedback components. It is used as the basic element in the performance of various mathematical operations such as addition, subtraction, multiplication, integration, differentiation and logarithms.
A basic schematic diagram of a conventional op amp is given in FIG. 1. A pair of transistors Q1,Q2 are differentially connected, dividing the current from a current source I1 which in turn is supplied from a positive voltage bus V+. The inverting input is provided at the base of Q1, and the non-inverting input at the base of Q2.
Equal currents are drawn through the collector-emitter circuits of Q1 and Q2 by a diode-connected transistor Q3 that is connected to Q1, and another transistor Q4 which mirrors the current through Q3 and is connected to Q2. The emitters of transistors Q3 and Q4 are connected to a negative voltage bus V- through respective equal value transistors R1,R2.
An output is taken from the collector of Q4 to the base of a second gain stage transistor Q5, which in turn feeds an output stage 2. An output terminal 4 is taken from the output stage, and supplies an output voltage V.sub.o.
A current source transistor Q6 supplies current to the differentially connected input transistors Q1,Q2. Since the two input branches are balanced and carry equal currents, the inverting input voltage tracks the non-inverting input voltage. Various input formats can be applied to the op amp in conjunction with a feedback circuit between the output terminal 4 and the inverting input to perform the various op amp functions, in a well-known manner.
When feedback is applied, however, the apparent symmetry between the inverting and non-inverting inputs is destroyed, resulting in a high impedance at the non-inverting input and a low impedance at the inverting input. The result is that applications centered about shunt feedback must be inverting only, and applications centered about series feedback must be non-inverting only. Because of this limitation truly differential circuits, such as instrumentation amplifiers, are usually constructed from multiple op amps and additional external components. With such an approach, however, it is difficult to maintain true symmetry without a requirement for matched amplifiers, extensive trimming and complicated frequency compensation techniques. Furthermore, the circuitry required to implement some of the op amp applications is complex, and it would be desirable to have a way of simplying it.